Composite alloy for high temperature applications

ABSTRACT

AN ANISOTROPIC POLYPHASE COMPOSITE ALLOY STRUCTURE HAVING GOOD HIGH TEMPERATURE TENSILE STRENGTH AND RESISTANCE TO CORROSION IS PROVIDED. THE COMPOSITE ALLOY CONSISTS ESSENTIALLY IN ATOMIC PERCENT OF 30-35% NICKEL, 30-35% ALUMINUM, 27-32% CHROMIUM AND 2.0-7.0% MOLYBDENUM AND HAS BEEN SUBJECTED TO UNIDIRECTIONAL SOLIDIFICATION TO FORM BODIES HAVING A MORPHOLOGY OF HIGH STRENGTH   ALIGNED PLATES OF CR(MO) EMBEDDED IN A NIAL MATRIX PHASE.

Jan. 1, 1974 J, WALTER EIAL 3,782,928

COMPOSITE ALLOY FOR HIGH TEMPERATURE APPLICATIONS Filed Nov. 8. 1972 3 Sheets-Sheet 1 NiAl X DENDRITES 0 COMPOSITE 9o 0 CELLS M0 [0 20 30 40 Cr ATOMIC PERCENT Or I n ve n to vs: John L. Walter, Harvey .65. Cline,

AR, Their Attorney.

COMPOSITE ALLOY FOR HIGH TEMPERATURE APPLICATIONS Filed Nov. 9, 1972 3 Sheets-Sheet 2 34$67896 M%Mo J. L. WALTER ETA!- Jan. 1, 1974 Their Attorney.

Jan. 1, 1974 J WALTER ETAL 3,782,928

COMPOSITE ALLOY FOR HIGH TEMPERATURE APPLICATIONS Filed Nov. 8, 1972 3 Sheets-Sheet 3 I h'\/ e n tor-1s: John L. Wat/ fem Hdr-vey'E. Cline,

Unitcd States Patent 3,782,928 COMPOSITE ALLOY FOR HIGH TEMPERATURE APPLICATIONS John L. Walter, Scotia, and Harvey E. Cline, Schenectady, N.Y., assignors to General Electric Company Continuation-impart of abandoned application Ser. No. 97,917, Dec. 14, 1970. This application Nov. 8, 1972, Ser. No. 304,614

Int. Cl. C22c 19/00 US. Cl. 75-134 F 5 Claims ABSTRACT OF THE DISCLOSURE An anisotropic polyphase composite alloy structure having good high temperature tensile strength and resistance to corrosion is provided. The composite alloy consists essentially in atomic percent of 30-35% nickel, 30-35% aluminum, 27-32% chromium and 2.0-7.0% molybdenum and has been subjected to unidirectional solidification to form bodies having a morphology of high strength aligned plates of Cr(Mo) embedded in a NiAl matrix phase.

This is a continuation-in-part of application Ser. No. 97,917, filed Dec. 14, 1970, now abandoned.

FIELD OF THE INVENTION The present invention relates to new and useful alloys, and more particularly to alloys containing eutectic systems for high temperature structural applications in corrosive atmospheres.

BACKGROUND OF THE INVENTION Unidirectional solidification of eutectic alloys is known, for example, from Pat. No. 3,124,452 of Kraft. Directionally solidified eutectic alloys have distinct advantages over ordinary composites for long-term high temperature structural applications. It is known that changes in the solidification rate and additions of third elements can modify the microstructures of binary eutectics to change the microstructure from lamellar to fibrous morphology. The eutectic NiAl-Cr system has been previously described as having excellent mechanical properties, for example, in the literature references: J. L. Walter and H. E. Cline, Met. Trans 1, 1221 (1970); and J. L. Walter, H. L. Cline and E. F. Koch, TMS-AIME, 245, 2073 (1969). However, while the known alloy eutectic NiAl-Cr has good oxidation resistance, its strength, without modification, is not sufficiently high for many high temperature structural applications such as gas turbine blades.

SUMMARY OF THE INVENTION By virtue of the present invention, enhanced physical properties are achieved subsequent to directional solidification in alloys of the NiAl-Cr system by the additions of specifically defined amounts of molybdenum.

The microstructure of the compositions of the invention is that of a NiAl matrix phase and a dispersed phase of chromium-molybdenum solid solution alloy plates, hereinafter shown by the symbol Cr(Mo), which is aligned substantially parallel to the direction of solidification and embedded in the NiAl matrix phase. Thus, at elevated temperatures of 1000 C., the preferred alloy containing 5.0 a/o molybdenum, 33 a/o nickel, 33 a/o aluminum and the balance chromium achieves an ultimate tensile strength of about 51,000 p.s.i.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be better understood from the following description taken in conjunction with the accompanying drawings wherein:

FIG. 1 is a photomicrograph (700x) showing a transverse section of a known NiAl-Cr eutectic alloy composition directionally solidified at the rate of 4 inch per hour;

FIG. 2 is a ternary phase diagram illustrating the compositions of alloys of the present invention and indicating some of the microstructures obtained as a function of the alloy compositions indicated in the diagram;

FIG. 3 is a graph illustrating the ultimate tensile strength at an elevated temperature of 1000 C. in alloys according to FIG. 2 as a function of the molybdenum content of the alloy;

FIG. 4 is a photomicrograph (1400X) of a NiAl-Cr Mo) ingot containing 3.0 a/o molybdenum directionally solidified at inch per hour; and

FIG. 5 is a photomicrograph (530x) of a transverse section of a NiAl-Cr(Mo) ingot containing 39.0 a/o chromium, 60 a/o NiAl and 1.0 a/o molybdenum and illustrating the microstructure of an alloy which is outside the scope of the present invention.

In the prior art illustration of FIG. 1, there is shown a transverse section through a directionally solidified composite stucture of the known alloy NiAl-Cr system which alloy contains 66 a/o NiAl and 34 a/o chromium. The structure of the alloy contains chromium rods or fibers as illustrated. The rods are well aligned except in the vicinity of the gain boundaries.

To improve the high temperature properties of the prior art alloy of FIG. 1, various percentages of molybdenum were added to replace some of the chromium. The compositions of the ternary NiAl-Cr(Mo) alloys and classification of their microstructures are illustrated in FIG. 2. Among the alloys having the desired microstructure of substantially aligned plates, the particular compositions are those consisting essentially in atomic percent as follows:

In alloys containing 66 a/o NiAl and up to 9.0 a/o molybdenum, structures of the NiAl-Cr(Mo) alloys at Warious atomic percent of molybdenum were investigated.

Above 0.6 a/o molybdenum, a transition in microstructures from rod grains to faceted rod and plate grains takes place with increasing molybdenum content. As the molybdenum content increases above 0.6 a/o, the morphology changes so that the number of faceted rods decreases and the plates become longer. As shown in FIG. 3, at 2.0 a/o molybdenum, a substantial increase in ultimate tensile strength at 1000 C. is observed and the optimum strength of about 51,000 p.s.i. is obtained when the NiAl-Cr alloy contains about 5.0 a/o molybdenum. A typical illustration of the desired microstructurc is shown in FIG. 4, Le. an alloy containing 3.0 a/o molybdenum, wherein the plates are generally straight and uniformly spaced. The spacing of the plates does not alter with increasing amounts of molybdenum. Beyond 7.0 a/o molybdenum, the microstructure begins to form the less desirable radial cell pattern, which results in reduced strength at high temperatures.

Another important parameter necessary to achieve the desired microstructures and to obtain the maximum strength is the solidification rate which should be no greater than /2 inch per hour, and preferably about inch per hour. When the solidification rate is too rapid, i.e. greater than /2 inch per hour, dendrites of the dis- I 3 persed phase occur similar to those shown in FIG. 5, causing a reduction in the tensile strength.

- Our invention is further illustrated by the following examples:

EXAMPLE I In order to prepare the preferred alloy of the present invention, consisting essentially of 29.0 a/o chromium, 33.0 a/o aluminum, 33.0 a/o nickel and 5.0 a/o molybdenum, these ingredients were first vacuum melted and then cast, under argon, into a inch diameter copper mold.

The cast ingots were remelted under argon in a vertical Bridgeman-type directional solidification apparatus. The melt was contained in an alumina crucible which rested on a water-cooled copper base to provide the necessary temperature gradient for directional solidification. The crucible containing the melt was Withdrawn from the hot zone of the apparatus at the rate of A inch per hour to provide the desired solidification rate of inch per hour. Test bars were taken from the ingot so solidified in a direction parallel to the solidification direction. As FIG. 3 shows, the composition of Example I has an ultimate tensile strength of approximately 51,000 p.s.i. when tested at 1000 C.

EXAMPLE II Following the procedure of Example I, alloys were melted and directionally solidified from the following compositions consisting in atomic percent of:

TABLE 2 Percent Composition Ni A1 Cr M 4 is not significant. The optimum c0mpo'sition. conta ining 5.0 a/o molybdenum appears as a maximum point on the graph having an ultimate tensile strength at 1000 C. of 51,000 p.s.i.

It will be obvious to those skilled in the art upon reading the foregoing disclosure that many modifications and alterations in the method steps and in the specific composition may be made within the general context of the invention, and that numerous modifications, alterations and additions may be made thereto within the true spirit and scope of the invention as set forth in the appended claims. Vim} We claim:

1. A directionally solidified cast anisotropic body consisting essentially in atomic percent of 30-35% nickel, 3035% aluminum, 2732% chromium and 2.0-7.0% molybdenum, said body having a dispersed phase of chromium-molybdenum solid solution alloy plates embedded in a nickel-aluminum matrix phase.

2. The body of claim 5, wherein said body consists essentially in atomic percent of about 33% nickel, 33% aluminum, 29% chromium, and 5% molybdenum.

3. A method of forming an anisotropic polyphase alloy body having improved high temperature strength and resistance to corrosion which comprises the steps of (a) providing a melt of an alloy consisting essentially in atomic percent of 30-35% nickel, 3035% aluminum, 27-32% chromium and 2.07.0% molybdenum, and

(b) casting and directionally solidifying the melt to form a dispersed phase of chromium-molybdenum solid solution alloy plates embedded in a nickel-aluminum matrix phase.

4. The method of claim 3, wherein the rate of directional solidification is no greater than V2 inch per hour.

5. The method of claim 3, wherein said melt consists essentially in atomic percent of about 33% nickel, 33% aluminum, 29% chromium and 5.0% molybdenum.

References Cited UNITED STATES PATENTS 3,564,940 2/1971 Thompson et a1. --171 RICHARD O. DEAN, Primary Examiner US. Cl. X.R. 

